Welcome to Unreal Forums @Flamey_8309
Just like @kaiagan1 said, your reference is not valid. If you would be kind enough to show how you’re setting up said reference, we might be able to point out a fix right away.
Right now you are trying to read value of “Position” int variable inside your First Person Character but engine doesn’t know where to look because you reference doesn’t point to anything (is null or empty if you will).
In the meantime I invite you to read below: